The present invention relates to a microwave integrated device and, more specifically, to a microwave integrated circuit device of a monolithic structure in which a transistor amplifier and a transmission line are formed on a single semiconductor substrate.
In a microwave integrated circuit (MIC) used in a microwave band and a millimeter wave band, especially in a monolithic microwave integrated circuit (MMIC) wherein a bipolar transistor is used for an active device and both a transistor amplifier and a transmission line are formed on a single semiconductor substrate, the transmission line is generally constituted of a microstrip line and a coplanar line.
FIG. 1 is a plan view of the arrangement of a prior art MMIC using a microstrip line.
As illustrated in FIG. 1, a multi-emitter transistor having a collector region 103, a base region 104 and an emitter region 105 is formed, as a bipolar transistor 102, on a semiconductor substrate (e.g., GaAs substrate) 101.
The transistor 102 serves as a base-grounded amplifier, and its emitter electrode is connected to an input signal line 108, and its collector electrode is connected to an output signal line 109.
Base electrodes of the transistor 102 are connected in common by a drawing line 110, and both ends of the line 110 are connected to a grounded conductive layer (not shown) formed on the undersurface of the semiconductor substrate 101 through via holes 111 and 112, respectively.
Since, in the arrangement of the prior art MMIC, a line extending from the base electrode and the grounded conductive layer is long, a parasitic inductor (hereinafter referred to as a ground inductor) L is formed on the line between the base electrode and grounded conductive layer (GND).
The value of ground inductor L is generally small and thus causes no specific problem in a frequency region of several gigahertz (GHz). In a high-frequency region of 10 odd gigahertz (GHz), however, the gain of the transistor 102 will be decreased.
If, in particular, the transistor 102 is constituted as a multi-emitter one, as shown in FIG. 1, with a view to increasing in output, the transistor 102 is expanded up and down in FIG. 1, or in a direction perpendicular to a transmission direction of a signal which flows from the emitter electrode to the collector electrode and thus the drawing line 110 is lengthened. By the multi-emitter, a low impedance of the transistor will be brought in addition to an increase in the value of ground inductor L, and thus susceptible to the ground inductor; therefore, the gain of the transistor is greatly lowered.
If a coplanar line is used as a transmission line in place of the microstrip line, the base electrode is connected to a ground conductor provided outside a transistor region on the semiconductor substrate without using any via holes. In this case, too, the problem of the decrease in gain occurs and if, in particular, the transistor is formed as a multi-emitter one, the problem becomes more serious.
In the foregoing prior art MMIC using a microstrip line or a coplanar line as a transmission line, the gain of the transistor is lowered by the ground inductor of the transistor in a high-frequency region of 10 odd megahertz (MHz). If, in particular, the transistor is constituted as a multi-emitter one to increase in output, its gain is greatly decreased, and thus the multi-emitter transistor cannot be taken advantage of.